1. Field of the Invention
This invention relates to switching voltage converter systems for converting a D.C. input voltage to a different D.C. output voltage. In particular, this invention relates to switching voltage converter systems employing semiconductor switches.
2. Background of the Invention
Switching voltage converter systems are employed in many application and, in particular, are employed in D.C. applications to convert an input voltage V.sub.IN to a different D.C. output voltage V.sub.OUT by means of a switching action in the system. In one type of converter system, the voltage change results from the induced voltage in an inductor due to the time variation of the current through the inductor due to the switching action. The operation of such switching converters is well known in the art with the basic configuration of a prior art switching converter system for stepping up the voltage being shown in FIG. 1.
The switching means employed in modern switching voltage converter systems is commonly a semiconductor switch such as a MOSFET, JFET or bipolar transistor or similar semiconductor device. Such semiconductor switches typically employ appropriate switching logic for turning the switch on and off as desired by applying the appropriate drive signal to the semiconductor device. A step up switching voltage converter system employing an n-channel MOSFET switch and suitable switching logic for driving the switch gate is shown in FIG. 2. In various applications the semiconductor switch and the switching control logic may be discrete circuits or form a part of a single monolithic IC semiconductor chip.
The efficiency of switching voltage converter systems is, in general, dependent upon the power losses in the circuit elements. In particular, power losses in the switch can have a significant effect on the efficiency of such converter systems. Where the switch of the system operates in a normal voltage range, the power loss in the switch will in general be lower where the forward voltage drop across the switch is minimized. The specific reduction in power loss by reducing the voltage drop across the switch and the corresponding increase in efficiency depends on the specific type of semiconductor device employed as the switch and the specific voltage range at which the system is operating.
The forward voltage drop across a semiconductor switch will typically be dependent upon the characteristics of both the switch device itself and the drive signal applied to the switch. In many prior art switching voltage converter systems, the switching logic which supplies the switch drive signal is powered by the input voltage V.sub.IN. For example, a MOSFET switch driven from the input voltage V.sub.IN supplied to the switching logic is shown in the prior art switching converter system of FIG. 2.
For a given operating region of a MOSFET switch, such as the switch shown in FIG. 2, the voltage drop across the switch can be reduced by increasing drive voltage between the gate and source (V.sub.GS) of the switch when it is on, the specific relationship depending on the particular switch design and operating range, and other device characteristics. This is also true for other FET switches and bipolar transistor switches (although in the latter case the voltage drop is reduced by increasing the current drive signal to the transistor base by increasing the voltage). The device characteristics of the semiconductor switch, although affecting the voltage drop across the switch, are generally not under the control of the circuit designer as this depends on the particular fabrication process used for the specific semiconductor switch. However, one device characteristic which typically is under the designers' control is the size of the transistor switch, with the voltage drop across the switch in general decreasing with increasing semiconductor switch size. More specifically, for a MOSFET switch, the voltage drop across the switch will be inversely proportional to the ratio of the device channel width to length. Making a switch very large is however undesirable for economic and space considerations.